1. Field of the Invention
The invention relates to an ink composition that is mainly used for ink jet recording, and an ink jet recording method using the ink composition.
2. Description of the Related Art
Various methods are known for forming an image on a recording medium such as paper based on an image data signal, for example, an electrophotographic method, a sublimation-type thermal transfer method, a melt-type thermal transfer method, and an ink jet recording method. The electrophotographic method requires a process of forming an electrostatic latent image on a photoreceptor drum by charging and light exposure, and a system therefore becomes complicated, resulting in increased production cost. The thermal transfer method can be conducted by an inexpensive apparatus, but requires use of ink ribbons, leading to increased running cost and generation of waste.
The ink jet recording method can also be conducted by an inexpensive apparatus. Moreover, an image is directly formed by jetting ink only to regions of a support which are to be image regions, and the ink can therefore be efficiently used, resulting in reduced running cost. Further, ink jet recording apparatuses are not noisy. Accordingly, the ink jet recording method is an excellent image forming method.
There is a demand for an ink composition, which is curable by irradiation of active radiation such as ultraviolet rays with high sensitivity to form high-quality images (radiation-curable ink composition), which can be used for an ink composition for ink jet recording.
Imparting a radiation-curable ink composition with high sensitivity-enables a high curing property to be obtained with respect to irradiation with active radiation. Consequently, various advantages are attained including reduced electric power consumption and extension of equipment service life due to diminished load on a radiation generator and, moreover, suppression of vaporization of uncured low molecular weight matter and suppression of reduction in the strength of formed images. In addition, because high sensitivity in a radiation-curable ink composition enhances the strength of a cured film, an image region formed from such an ink composition is provided with high strength and durability.
As a UV-curable ink composition, for example, an ink composition using, in combination, mono-functional or multi-functional monomers having different functional groups from each other has been proposed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 5-214280). Further, another ink composition has been proposed, which is a radiation-curable composition containing a multi-functional acrylate (for example, see JP-A No. 8-41133). These ink compositions are excellent in curing speed and can provide images free of ink bleeding, but are problematic in terms of lowering adhesiveness (adhesion) to a recording medium due to volume shrinkage upon curing.
Regarding the adhesiveness of a UV-curable ink composition to a recording medium, a radiation-curable ink jet ink composition that uses N-vinyl lactam as a component promoting adhesiveness to the recording medium and flexibility after curing has been disclosed (for example, see JP-A No. 2004-514014). However, there are concerns that N-vinyl lactams represented by N-vinyl caprolactam may generate surface tackiness of cured ink images or cause so-called “leaching” in which low molecular weight components bleed out of the surface due to their low co-polymerizable property with widely-used radical polymerizable monomers such as acrylates. Further, there are a variety of problems such as the reactivity of N-vinyl lactams leading to poor storage stability due to, for example, an increase in ink viscosity, or N-vinyl lactams being unsuitable for white or light color inks from the viewpoint of color reproducibility because of coloring after curing.
As discussed above, an ink composition that exhibits excellent stability for use in an ink jet apparatus and excellent color reproducibility in cured ink images while maintaining excellent adhesiveness to a recording medium, excellent curing sensitivity, and excellent image strength is sought after; however, at present, such an ink composition is not available.